Process for the production of carbon filaments from coal tar pitch

ABSTRACT

CARBON FILAMENTS ARE MADE FROM SPECIALLY TREATED HIGH TEMPERATURE COAL TAR PITCH. THE PITCH IS FILTERED, HEAT TREATED AND ITS LOW MOLECULAR WEIGHT COMPONENTS ARE REMOVED BY DISTILATION. THE TREATED PITCH IS SPUN INTO FILAMENTS, THE FILAMENTS PARTIALLY OXIDIZED AND THEN CARBONIZED UNDER CAREFULLY REGULATED CONDITIONS TO YIELD FILAMENTARY CARBON HAVING A TENSIL STRENGTH OF MORE THAN 100,000 P.S.I. THE FILAMETNS MAY BE GRAPHITIZED.

United States Patent 3,718,493 PROCESS FOR THE PRODUCTION OF CARBONFILAMENTS FROM COAL TAR PITCH Louis A. Joo, Johnson City, John A. McKee,Elizabethton, and Frederick L. Shea, Johnson City, Tenn., assignors toGreat Lakes Carbon Corporation, New York,

No Drawing. Original application June 4, 1968, Ser. No. 734,257, nowPatent No. 3,595,946. Divided and this application Oct. 28, 1970, Ser.No. 34,896

Int. Cl. C08h 13/00; C0911 3/24 US. Cl. 106--273 R 1 Claim ABSTRACT OFTHE DISCLOSURE CROSS-REFERENCES TO RELATE APPLICATIONS This applicationis a division of application S.N. 734,- 257, filed June 4, 1968, now US.Pat. No. 3,595,946.

Carbon fibers have been produced from organic polymer fibers such asrayon or polyacrylonitrile by subjecting the latter to a regulatedcarbonization, preceded, in the case of fusible polymers, by acontrolled oxidation to an infusible state. A difiiculty with this typeof approach has been the low yield of carbon available from syntheticpolymers.

Satisfactory carbon fibers have also been made from molten decompositionproducts of synthetic polymeric materials such as polyvinylchloride,polyvinyl acetate, and from fusible carbon-yielding materials like blownasphalt and petroleum pitch. In this type of process, the fusiblematerial is spun into fibers, oxidized and carbonized to yield carbonfibers possessing strengths of over 100,000 p.s.i. In some cases, it hasbeen found necessary to condition the starting material by heattreatment to raise its molecular weight and render it more amenable tothe further transformations just mentioned. In the case of coal tarpitch however, these measures have failed and the art does not provideany successful method for the satisfactory spinning of pitches of coaltar origin and for their subsequent conversion into useful carbon fibershaving properties comparable to those possessed by carbon fibers fromother raw materials.

It is an object of this invention to provide easily spinnable materialsfrom coal tar pitches capable of being transformed into useful carbonfibers. It is a further object to provide a method by which saidmodified coal tar pitches can be oxidized and carbonized satisfactorily.Still another object is to provide from common industrial coal tarpitches carbon fibers with tensile strengths in the class of 100,000p.s.i. and which also possess to a sufficient degree the other desirableproperties generally associated with such refractory material.

SUMMARY OF THE PROCESS These and other objects which shall becomeevident from the detailed description of the invention, have beenaccomplished by removing from a coal tar pitch substantially all of thematerial therein which is insoluble and remains as a second phase at thespinning temperature. Before or after this operation, the pitch is heattreated and distilled in order to increase its average molecular weightby polymerization and by removal of low molecular weight componentsformed or already present in the pitch. Various oxidizing,dehydrogenating and polymerizing agents may be employed in a number ofmanners to expedite this process. The treated pitch is then melted andspun into air, the resulting filament being stretched and woundcontinuously in conventional textile manner. The pitch filament isoxidized in one or more stages in an oxidizing medium and thencarbonized at a temperature in the vicinity of 1000 C. in an inertatmosphere. If desired, the resulting carbon filament may be graphitizedby heating in an inert atmosphere at elevated temperatures.

DETAILED DESCRIPTION The raw materials from which the fibers of theinvention are spun consist of commercially available high temperaturecoal tar pitches having a ring-and-ball softening point (A.S.T.M.method) within the range of to 250 C. Incidentally, all softening pointsin the text of this application are ring-and-ball softening points andall percentages are on a weight basis. Among the usable coal tarpitches, those preferred have a softening point within the range of to200 C.

A critical operation in the transformation process of the invention isthe removal of the quinoline insolubles from the coal tar pitchselected. Quinoline insolubles represent material which is not solublein the pitch at spinning temperature or, in other words, Which forms anundesirable second phase. This removal is generally done before heattreating and distilling the pitch. This sequence of operations howeveris not binding, especially when a pitch of low quinoline insolublecontent is employed. To accomplish the removal of the undesirablefraction, the pitch may be diluted in an appropriate solvent, filteredor centrifuged and recovered. The solvents usable for this purpose aregenerally speaking any aromatic liquid having a boiling point or rangebetween about 200 and 400 C., provided that the major liquifiableportion of the coal tar pitch is soluble in it. Said liquid should alsobe removed from the filtered pitch solution at temperatures preferablynot exceeding 300 C., either at atmospheric pressure or under reducedpressure. Examples of usable liquids which fit these specifications arelight creosote oil, anthracene oil, phenanthrene, quinoline, highlyaromatic petroleum fractions and the like.

The dilution of the pitch and the subsequent filtration can be carriedout at any temperature Within a range g0- ing from the softening pointof the pitch to about 300 C. Proportions of solvent to pitch vary withthe viscosity of the substance at the temperatures selected. In general,a dilution of 1:1 at a temperature of about 200 C. has been foundconvenient. The diluted pitch is then filtered to remove the undissolvedmaterial. This may be accomplished in a number of manners with knownequipment, provided of course that the foraminous member of thisequipment can withstand the filtration conditions. Fritted glass andporous stainless steel septums having openings of about 10 microns inaverage diameter are satisfactory. The filtration process may befacilitated and improved in a conventional manner by additions of knownfilter aids to the pitch liquid. The number of actual filtrations andtheir timing may vary according to several factors such as viscosity ofthe pitch liquid, amount and nature of undissolved solids, temperature,pressure and the like. Sufiice it to say here that at least onefiltration is required for the purpose of this invention and that it maybe carried out on a liquid solvent diluted or undiluted pitch at anystage before the actual passage of the pitch through thefilament-forming spinneret. After completion of the filtration, thesolvent is removed by evaporation preferably under reduced pressure atabout 150 C., in any event, at a temperature not exceeding 300 C. Asmentioned previously, the separation of the undesirable second phase maybe accomplished by centrifugation rather than filtration, although thelatter technique is preferred in most instances.

The coal tar pitch to be used for spinning filaments is heat treated anddistilled to improve its molecular weight range. Either or both of thesetreatments may be carried out, as indicated earlier, before, after orbetween filtrations. The pitch is distilled at a temperature within therange of about 280 to 305 C. to remove its lower molecular weightcomponents. This may be accomplished by any conventional methodincluding distillation under re duced pressure, steam distillation andso on, using any conventional equipment such as a molecular still andthe like, provided the temperature limits are respected. The pitch isheat soaked at similar temperatures for a period of about 10 to 100hours or more under pressures which may range from less than oneatmosphere to more than one atmosphere. The actual length of this heattreatment depends of course on the nature of the pitch components aswell as on other factors. Polymerization can also be favored byincorporating within the pitch various oxidizing, dehydrogenating andpolymerizing agents which may shorten the heat treatment or lower theoperating temperature. Among suitable materials which may accomplishthese ends are included organic and inorganic peroxides and high boilingnitroaromatie compounds such as nitronaphthalene and2,4-dinitrochlorobenzene, and the like. In any event, the heat treatmentmust be carried in such a manner and under such conditions, within thelimits already described, that there is produced a spinnable pitchhaving a softening point or range within 230 and 320 C., and a quinolineinsoluble content of less than 2%. Preferred pitches within these limitsare those which soften between 240 and 260 C. and contain no secondphase at spinning temperatures or below.

The heat treated coal tar pitch prepared in the manner described is thenspun into a continuous filament through a nozzle or spinneret with aninternal diameter appropriate for the thickness of filament required.For spinning, the pitch is melted at a temperature between its meltingpoint and about 300 C. and the melt is extruded through the spinneretsorifice by sufiicient nitrogen pressure to achieve a satisfactory rateor by other conventional means such as a metering pump, a piston and thelike. The actual pressure and temperature used depend on the propertiesof the heat treated pitch used as well as upon each other. In thisrespect, it has been found that a pitch of the type prescribed by thisinvention can be spun satisfactorily under a driving N pressure of about80 p.s.i. through a perforated stainless steel septum with poresaveraging 10 microns or less and a spinneret orifice of 1.5 mm. inlength and 0.3 mm. in diameter. More than one orifice may of course beemployed. The filament produced emerges in air and is stretched andtaken up on a conventional textile winder at a fixed speed, e.g. at 250to 300 meters per minute. Filaments with diameters ranging from about 5microns and higher can thus be produced.

Extrusion of the filament at high temperature initiates an oxidationprocess which may be intensified by passing the filament through anoxidizing atmosphere for a length of time sufficient to create theinfusibility required by the subsequent carbonization treatment.Suitable oxidizing media for this purpose include air; ozone in air;oxygen blended with air, flue gases or inert gases; vapors or mists ofnitroaromatic compounds such as nitrobenzene, nitrophenol,alpha-nitronaphthalene, nitrotoluene, nitrochlorobenzene and the like;oxidizing gases such as sulfur dioxide, sulfur trioxide, nitric oxideand the like. Alternately, the filament may be cooled to a temperaturebelow its fusing point and then passed through liquid oxi dizing bathsof the above mentioned nitroaromatics or of other oxidizing liquids suchas nitric acid, sulfuric acid, chromic acid, permanganate solutions andthe like.

These various oxidizing treatments may be carried out in a continuousmanner on the filament emerging from the spinning machine or they may beapplied to batches of filament wound into packages. In such aneventuality, the support of the filament package must be of such natureand/or construction that it yields or collapses as the wound filamentcontracts during the oxidation process.

Papers cylinders have been found useful in this function.

The oxidation of filament Wound in packages must follow a fairlycritical heating regime if the superimposed and adjacent loops offilament are not to fuse together. This regime will naturally vary withthe pitch, its previous oxidation history and the type and quality ofadditive present, if any. The best heating rates and soakingtemperatures for a given material are naturally difficult to determinesince the fusion temperature of the pitch changes as the oxidationproceeds. Nevertheless, it has been established that a heat treatedpitch of the type preferred, as described earlier, will yield filamentsthat are successfully oxidized by raising the temperature to C. in lessthan 15 minutes, a non-critical step; holding the filament at 100 forabout 20 hours; raising the temperatures from 100 to 195 C. at apreferred rate of about 5 C./hour; holding the filament at the lattertemperature for a period within the range of about 60 to about hours,the upper part of that range being preferred. It should be noted thatwith certain materials temperature increase rates of up to 10 C./hourmay be tolerated. In any event, the temperature at any time during theoxidation treatment should preferably be not higher than 10 C. below thesoftening point of the pitch at the given time. This batch typeoxidation is best carried out in a circulating oven through which passesa constant flow of air or oxygen containing gas, both fresh andrecycled, pre-heated at the desired temperature.

The oxidized filament may then be cooled to room temperature orsubjected immediately to carbonization. If cooling is elected, it shouldbe gradual, the to 100 C. step being accomplished at about the same rateas the reverse step previously carried out, with the last 100 to roomtemperature adjustment taking about three hours.

The oxidized pitch filament is then converted to a carbon filament. Thisis accomplished in an oven or kiln provided with means for allowingsweeping of the reaction area with an inert gas, e.g. nitrogen scouredthrough hot coke. When Wound filament in packages is treated, thepackages are placed in a sagger and, as the inert gas is allowed tosweep the package for example from bottom to top, the temperature of thekiln is raised according to the following typical cycle: from 30 to 100C., when necessary, at the rate of 10/hour; from 100 to 500 C. at 5/hourand from 500 to 1100 C. at 10/hour. Cooling to room temperature alsoshould be gradual, e.g. from 1100 to 30 C. in about 36 hours. As the 100to 500 C. temperature range is the most critical in the carbonizationprocess, special care need be exerted in controlling the heating ratethrough that range. As to the top carbonization temperature, it must benoted that useful carbon fibers may be produced from 700 C. foramorphous carbon filaments to as high as 2800+ C. if graphitic filamentsare desired. Heating rates and holding times are no longer criticalabout 1100 C.

A graphite filament may be conveniently prepared by heating a carbonfilament for about one hour in argon at above 1500 C.

Carbon filaments (1100 C.) of conventional textile lengths may beproduced from coal tar pitch by the method just described, having atensile strength of 80,000 to 130,000 p.s.i., a modulus of elasticitywithin the range of 4.5 to 5.2)(10 p.s.i., a volume resistivity inohm-inch of 1200 to 1600 and an apparent density of about 1.65

g./cc. Such filaments, and their graphitic counterparts, are eminentlysuited as substrate in vapor phase depositions such as manufacture ofboron filaments, heat resistant reinforcement in fiber-matrixcomposites, as well as in other similar applications where filamentarycarbon is conventionally and advantageously employed.

A better understanding of the process of the invention may be obtainedfrom the following example. This embodiment is provided for illustrativepurposes only and must not be construed as limiting the invention beyondthe scope of the claims that follow this specification.

EXAMPLE A coal tar pitch having a softening point of 187 C., AlliedChemicals Companys CP-275 grade 350, was dissolved at 180 C. in an equalweight of a light creosote oil fraction having a boiling range of 270 to315 C. for 88% of its content. The solution was filtered with DicaliteSpeedplus diatomaceous earth through a coarse 40 to 60p fritted glassplate, then through a 4.5 to 5.511. fritted glass plate. The filtratewas stripped to 130 C. under a pressure of 3 mm. 'Hg. The soluble pitchfraction was heat treated for hours at 280 to 305 C. pot temperature,under a 50 C. condenser in which about 17% of low melting solid fractionwas collected at 2 mm. Hg. The heat treated soluble pitch remainingafter this treatment had a softening point of approximately 256 C.

The modified coal tar pitch just described was spun through a spinnerethaving an orifice of 1.5 mm. in length and a diameter of 0.3 mm. Themolten pitch at 287 C. was driven through the spinnert by a nitrogenpressure of 110 psi. The resulting pitch filament had a final diameterof about 30 microns, when stretched and taken up on a paper cylinder atthe rate of 256 meters per minute.

The wound filament paper packages were then hung on graphite supports inan oven through which fresh and recycled air in a ratio of about 1:1 wasconstantly circulated while the temperature was raised and lowered in100 to 40 C. 2.0

The total residence in the oxidizing oven was thus 178.25 hours.

The oxidized filament packages were then placed in a stainless steelsagger on the same type of graphite hangers as used in the oxidationstep, and subjected to the following time-temperature regime while beingconstantly swept by nitrogen previously scoured through coke at processtemperature:

Hours 40 to 100 C., 10/hour 6 100 to 500 C., 5/hour 80 500 to 1100 C.,10/hour 1l00 to ambient temperature 3 6 The resulting carbonmonofilament was tested on an Instrom tester at a cro'sshead speed of0.2 in./min., using 1" gauge lengths. It was found to possess a tensilestrength of 127,400 p.s.i., an elongation at break of 2% and a modulusof elasticity of 5.1 X 10 p.s.i.-a1l these measurements being averagesof at least 6 independent determinations.

The carbon monofilament had an average diameter of 25 microns and, asmeasured on 0.125 inch long samples, a volume resistivity of 1419,uohm-inch with a variation of 15.0%.

There was thus produced by the method of this invention a carbonfilament from coal tar pitch, that has excellent mechanical andelectrical properties as compared to what was heretofore taught by theart. It is evident that these properties may be maximized by the manskilled in the art while remaining within the scope of the invention asdescribed in the foregoing specification and defined by the followingclaims.

We claim:

1. A preconditioned coal tar, fiber forming, spinnable pitch ofsoftening point about 230 to 320 C. containing less than 2 percent byweight of insoluble material at spinning temperature and below.

References Cited UNITED STATES PATENTS 3,070,449 12/ 1962 Davies et a1106-284 X 3,629,379 12/ 1971 Otani et a1. 26429 3,392,216 7/1968 Otaniet a1. 26429 JOAN B. EVANS, Primary Examiner US. Cl. X.R.

